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Replacing 30 year old 2" weathermatic valve

The valve is brass and the screws just crumble when we try to remove. The valve is threaded on each end and connected to steel pipe. I assume we have to cut off at least one end (maybe both). What valve should we buy and how do we install it?

If you have to cut both ends off you have my sympathy. Actually, even if you cut only one.
The steel pipe will have to be re-threaded or you will need to track it back to the joint and unscrew it there.
Assuming you can get to it and assuming you can get 30 year old pipe to unscrew.

Answering the question you asked: any 2" valve should work, brass orPVC. This is the least of your worries.
If you are familiar with re-threading the pipe and have the tools to do so, great.
If not you can rent them and practice on a different piece of pipe. Practice a lot.

Cut the pipe back far enough to allow for installation of a coupler and new pipe pieces. You can stay with all steel/brass or convert that part to PVC.

This might be one of those times where you want a professional involved.

If the valve is so old that the cover bolts are brass instead of stainless steel, it might never be repairable. That said, I have never ever seen a brass Weathermatic valve that can't be repaired, and at a lower cost than other brass valves.

Replacing 30 year old Weathermatic valve

I can't repair the valve. First, as I said the cover's bolts just crumble when we try to remove them. Second, the port under the solenoid is broken. I.e., the little nipple has come out of the valve, leaving a hole that the solenoid can't close.

We can't find any markings to tell us the model of the valve to get replacement parts, if we could get the top off. It just says Weathermatic, Dallas, Texas.

I have called a professional but out here all service companies have a long waiting list. (Oil boom plus a drought). I am going to try and plug the hole under the solenoid to get the valve to turn off so I can water my other zones. Any ideas?